The present invention relates to a process for the catalytic dehydrohalogenation of 1,2,4,-trichlorobenzene with hydrogen to form 1,2-dichlorobenzene over a platinum-spinel catalyst.
U.S. Pat. No. 2,866,828 discloses the dehalogenation of trihalogenated benzenes over various catalysts, aiming predominantly at the 1,3-dihalobenzene. In Example 4 of this U.S. Patent, a 1,2,4-trisubstituted benzene is dehalogenated over a platinum-charcoal catalyst. However, in this reaction, the industrially unimportant, 1,2,4-tribromobenzene is used in only one short test run; in addition, a high percentage of monobromobenzene is formed.
U.S. Pat. No. 2,943,114 likewise describes the preparation of m-dichlorobenzene by reference to short test runs. Using various catalysts, only low selectivity of dehalogenation is observed on the whole. In Example 5 of this U.S. Patent, a platinum/aluminium oxide catalyst in used without the experimental results being reported.
In U.S. Pat. No. 3,595,931, complete dehalogenation which, according to Example 5 of this U.S. Patent, achieves a 70% conversion is aimed at by means of a platinum/aluminium oxide catalyst doped with potassium hydroxide.
U.S. Pat. No. 2,886,605 describes the dehalogenation of aliphatic and aromatic halogen compounds. The reaction is carried out in a fluidized bed over an aluminium oxide/cuprous chloride catalyst. According to the short test runs described only a small amount of dichlorobenzenes, whose main component is the m-isomer, are obtained in the reaction mixture.
However, to carry out a dehydrohalogenation from industrial aspects, a high selectivity of the process and a long catalyst life in combination with almost undiminished activity are desired. Surprisingly, it has now been found that in this respect the selection of spinels as carriers plays a dominant role. Such catalysts used according to the invention show only a small degree of deactivation even in long-term experiments of over 2,000 hours.